Militants took a town an hour from Baghdad that is home to four natural gas fields on Thursday, another gain by Sunni insurgents who have swiftly taken large areas to the north and west of the Iraqi capital.

Iraq's presidency said a session of parliament would be held on July 1, the first step to forming a new government that the international community hopes will be inclusive enough to undermine the insurgency.

The overnight offensive included Mansouriyat al-Jabal, home to the gas fields where foreign companies operate, security forces said. The fighting threatens to rupture the country two and a half years after the end of U.S. occupation.

The insurgents, led by the hardline Islamic State in Iraq and Syria (ISIS) but also including other Sunni groups blame Prime Minister Nuri al-Maliki for marginalizing their sect and he is fighting for his job.

Three months after elections, a chorus of Iraqi and international voices have called for the government formation process to be started, including Iraqi's most influential Shi'ite Grand Ayatollah Ali al-Sistani.

The presidency issued a decree on Thursday for a parliament session on July 1, state television said. Parliament will then have 30 days to name a president and 15 days after that to name a prime minister although the process has been delayed in the past, taking nine months to seat the government in 2010.

Maliki has dismissed the call of mainly Sunni political and religious figures, some with links to armed groups fighting Maliki, for a "national salvation government" that would choose figures to lead the country and, in effect, bypass the election.

Northern Iraq's largest city Mosul fell to Sunni insurgents on June 10 and they took Tikrit city two days later. Kurdish forces moved into Kirkuk on June 11 and now control the oil city.